This project involves the study of light-sensitive enzyme reactions in vertebrate retinal photoreceptor outer segments. The particular enzyme functions under investigation are rhodopsin kinase and cyclic nucleotide phosphodiesterase. Major emphasis in the past year has been placed on determining the initial rates of these reactions in order to ascertain whether they may be involved in visual excitation or in some later process, such as dark adaptation, and on determining whether there is a causal relationship between these two enzyme activities.